Heretofore, it has been common to employ separation membranes made of various ion exchange membranes in various fields. Also in polymer electrolyte fuel cells which have been actively developed recently, an ion exchange membrane as one of separation membranes is used as a polymer electrolyte membrane. The polymer electrolyte fuel cells are expected to be widely used for movable bodies such as automobiles, or as distributed power generation systems or cogeneration systems for home use, since their power density is high and their operating temperature is low, whereby downsizing is possible.
For polymer electrolyte fuel cells, a polymer electrolyte membrane having a thickness of from about 20 to 120 μm is usually used, and a cation exchange membrane made of a perfluorocarbon polymer having chemically stable sulfonic acid groups is used in many cases. When power generation is carried out, catalyst layers containing metal catalysts are bonded on both sides of the electrolyte membrane to prepare a membrane-catalyst layer assembly, and then gas diffusion layers made of e.g. carbon cloths or carbon papers are disposed on both outside surfaces to prepare a membrane-electrode assembly. Further, on both outside surfaces of the gas diffusion layers, electroconductive separators having gas channels formed are disposed respectively to form a minimum unit for power generation called a single cell. However, the voltage generated in the single cell at usual power generation is at most 1 V. Therefore, in order to obtain a practical voltage, a plurality of such single cells are laminated and used as a stack.
The above catalyst layers are formed by applying a dispersion having, as main solid components, carbon having a metal catalyst supported thereon and a polymer electrolyte resin (ion exchange resin) dispersed in a dispersion medium, directly on a polymer electrolyte membrane, or applying the dispersion on a separately prepared substrate and subsequently transferring it on a polymer electrolyte membrane by e.g. hot pressing.
However, the polymer electrolyte membrane undergoes a dimensional change depending upon the water content. In addition, such a polymer electrolyte membrane is insufficient in mechanical strength. Therefore, when a membrane-electrode assembly is to be produced, positioning tends to be difficult in the production process, wrinkles tend to be formed in its production, or the polymer electrolyte membrane is likely to be torn. Further, even when the membrane-electrode assembly produced is free from e.g. wrinkles, it has a difficulty in handling during assembling a cell or stacking, or the polymer electrolyte membrane is likely to be torn during cell operation, and thus, reliability of the cell is not necessarily sufficient. Accordingly, it is desired that a membrane-electrode assembly has sufficient mechanical strength, chemical stability and dimensional stability.
As a method to solve the above-mentioned problems, it was proposed to use a membrane which is a polytetrafluoroethylene (hereinafter referred to as PTFE) porous membrane impregnated with a fluorinated ion exchange polymer having sulfonic acid groups (JP-B-5-75835). Further, a cation exchange membrane reinforced with a fibrillated, woven or non-woven perfluorocarbon polymer was also proposed (JP-B-6-231779). However, neither of them had a sufficient effect to suppress a stress which causes the ion exchange membrane to stretch when hydrated. Accordingly, a substantial dimensional change occurred, and the mechanical strength was insufficient.
Further, a method of introducing an electrolyte into a membrane substrate having perpendicular communicating pores with a diameter of approximately 8 μm (U.S. Pat. No. 4,673,624) or a method of introducing ion exchange groups into a membrane substrate having communicating pores with an area of from 0.2 to 30,000 nm2 which are perpendicular to the thickness direction (JP-A-2002-203576). However, such a membrane substrate has a restriction such that its variety is limited, and therefore there is such a problem that a chemically stable substrate can not necessarily be selected. Further, in a case where the pore diameter is small, there is a problem in production efficiency because it takes time and costs to form pores on a membrane substrate having a large-area as a practical size so as to secure a sufficient numerical aperture.
Further, it has been proposed to improve the handling efficiency at the time of assembling a cell or stacking, and further improve the strength in the peripheral portion of a membrane by providing a frame-like reinforcing film having its center cut away on the principal peripheral portion of a membrane-electrode assembly at which ion conductivity is not required (JP-B-3052536). However, there is a problem in productivity at bulk-production such that even when the frame-like film is provided on the peripheral portion, wrinkles are likely to be formed due to poor adhesion to the ion exchange membrane.